Gratings are arranged at the inlets to hydroelectric installations. An essential function of these gratings is fish protection. In their capacity as a mechanical barrier, the gratings are intended to prevent the fish from swimming into the turbine inlets in their downstream migration. Depending on the type of turbine and the size of fish, some of the creatures entering the turbine area are injured or killed. The smaller the gaps between the bars of the gratings, the better the fish are held back.
In the course of the approval proceedings for hydroelectric installations, very small grating spacings are often stipulated for the purposes of fish protection. The smaller the spacings of the gratings, however, the higher the hydraulic losses and the costs involved in cleaning and maintaining the gratings.
Typically, the gratings are arranged in an upright position at the entrances to the hydroelectric installations, i.e. in front view the bars of the gratings extend vertically. Taking account of the typical body shape of fish, gratings with horizontally extending grating bars have proved advantageous. These are increasingly being used at new hydroelectric installations but give rise to by no means negligible investment and maintenance costs.
From DE 38 31 099 A1, a fine grating is known in which a wire cable is braced between two rollers. This is not a fish protection cable screen; in particular, a grating of this kind is not suitable for mounting at an inlet of a hydroelectric installation.
In DE 44 13 841 A1 and U.S. Pat. No. 1,739,701 A, grating-like structures are described in which cables are braced in a frame, in order to cope with comparatively small bracing widths. These are not fish protection cable screens with braced cables for holding back and diverting fish.